Similar Protocols

Reproducibility Feedback

Share your feedback

Abstract

Iron is an essential micronutrient required for virtually all organisms. This fact is related to the ability of the transition metal to exist in two oxidation states, the reduced ferrous (Fe2+) and the oxidized ferric (Fe3+). Given the relative availability of aqueous iron (the element which constitutes ~5% of the earth’s crust) one is not surprised that iron is the most common prosthetic element in biology. Usually, fungi can uptake iron through receptor-mediated internalization of a siderophore or heme, and/or reductive iron assimilation (RIA) (Kosman, 2013). In this way, the uptake of iron in the absence or presence of the reducing agent ascorbic acid can be investigated by 59Fe uptake assays, as previously described (Eide et al., 1992). In the presence of ascorbic acid, the reductive-independent 59Fe uptake route is investigated. On the other hand, in the absence of ascorbic acid, the reductive-dependent 59Fe uptake route is stimulated. Using this strategy for the human pathogenic fungus Paracoccidioides species, the results showed that iron uptake by Pb01 in the absence of ascorbic acid was low, unlike what was observed for Pb18. These results suggest that only in Pb18 the iron uptake pathway is coupled to a ferric reductase (Bailão et al., 2015). In this protocol, we describe how to perform 59Fe uptake assays in Paracoccidioides species.

Grow the Paracoccidioides spp. cells (from a stock* grown in BHI with 4% glucose medium for at least 72 h at 36 °C) in 200 ml BHI with 4% glucose medium for 72 h at 36 °C with shaking (around 300 rpm). After this, collect 5 x 106 viable cells counted in a hemocytometer with Trypan blue solution** and transfer to 30 ml of SC medium with no supplementation or supplemented with 200 μM BPS or 10 μM FeCl3 for 24 h at 36 °C with shaking.*Note: This stock remains viable until seven days. After this, a new culture has to be initiated in tubes containing fresh BHI with 4% glucose medium.** Note: To count the viable cells, dilute the cells in 0.9% NaCl solution 100 times (it could be two 1:10 serial dilutions). After the dilution, get 10 μl of the cells and mix with 10 μl of Trypan blue solution. This corresponds to a 1:200 final dilution. Get 10 μl of this mixture and count the cells using a hemocytometer. The viable cells will not uptake the blue dye and will stay colorless.

Collect 20 ml of cells into a conical tube by centrifugation at 1,200 x g for 10 min at 4 °C and wash cells once with 1 mM EDTA in citrate uptake buffer.

Wash cells twice with citrate uptake buffer (pH 6.0).

Re-suspend cells to a final volume of 15 ml with citrate uptake buffer.

Transfer 7 ml of cells into a scintillation vial. You’ll need more than 7 ml of cells if you have more than two time points.

Incubate cells for 15 min at 36 °C with shaking prior to addition of 59Fe.

While shaking cells, take an aliquot (100 μl) from each scintillation vial and count the viable cells by a hemocytometer with Trypan blue solution. Note: This step is important to data normalization at the end of the experiment, since the data will be presented as pmol 59Fe/106 viable cells/time.

For reductase-independent 59Fe uptake, add 140 μl 1 M ascorbic acid in distilled H2O to each scintillation vial (20 mM final concentration). Note: The ascorbic acid solution should be made just prior to use.

To begin uptake, add 70 μl of 20 μM 59Fe solution to each of the scintillation vials.

At each time point (be sure to have a zero time point), stop shaker, carefully remove the scintillation vials, and pipet 1 ml of cells into 13 x 100 mm test tubes containing 3 ml of ice-cold 1x quench buffer on ice. Do this in triplicate for each time point (Figure 1).

Figure 1. Quenching uptake samples. At each time point 1 ml samples are withdrawn from the uptake vials and quenched in 3 ml ice-cold quench buffer. It is important to add the quench buffer immediately after collecting 1 ml sample.

For filtration and washing of cells, pre-soak filters in 1x quench buffer in a Petri dish.

Using the microfine forceps, carefully place 12 filters on the manifold (hold only the outer edges of the filters) (Figure 2). Place the top on the manifold and tighten it down with the blue plastic screw. Apply vacuum to the filter manifold using cold water to form a vacuum.

Figure 2. Placing filters on the Millipore manifold. Prior to initiating the 59Fe-uptake by your fungal samples, the filters are wetted in quench buffer and then placed on each of the manifold’s frits with the help of tweezers. The top on the manifold is then placed and tighten it down with the blue plastic screw. Apply vacuum to the filter manifold using cold water to form a vacuum.

When all the samples have been collected, wash quenched samples on the Millipore manifold (Figure 3).

Figure 3. Filtering quenched cell samples. After collecting and quenching all of the uptake samples, they are filtered and washed on the manifold for three times. It is important to wash filters in a series of # 1-12. Do not wash filter # 1 three times and then proceed to wash # 2 three times. If collecting more than the number of wells on the manifold, the first 12 filter set is removed for counting and replaced with fresh filters to continue the washing of additional samples.

Pour samples over each filter. Rinse the test tube once with 3 ml ice-cold 1x quench buffer. Pour that over each corresponding filter. Rinse each filter three times with 3 ml ice-cold 1x quench buffer. Wash filters in a series of 1-12. For example, do not wash filter in position 1 three times and then proceed to wash the filter in position 2 three times. Leave manifold under vacuum until you have filtered and washed all samples.

Leaving the manifold under vacuum, allow filters to dry (~30 sec) after the last wash.

Using the forceps, remove each filter by folding it in on itself (NEVER touch the middle or top of the filter) and placing it in a 12 x 75 mm glass test tube.

Continue with steps 11-15 until all of the samples have been processed.

Using a wooden applicator, gently push the filters to the bottom of the test tube.

Place the test tubes in the g counter carrier racks and count samples including 59Fe standards prepared in 12 x 75 mm test tubes. Use at least five points for standards, such as 20 μM, 10 μM, 1 μM, 0.1 μM and 0.05 μM (Figure 4).

Dispose of radioactive waste as per institutional environmental health and safety regulations.

Representative data

Figure 4. Representative data expected to obtain after the g counter reads. The standards are important to provide a calibration curve from which it will be possible to convert counts per minute (Cpm) to pmol 59Fe (top graphic). After normalization of the data with the 0 time point (fifth column), the pmol 59Fe is calculated based on the calibration curve and then it is possible to correlate the quantity of 59Fe (in pmol) 106 cells could internalize at a time, in this case 60 min (lower graphic).

To make a working stock of 20 μM 59Fe in ddH2O:
For an experiment with 2 time points and 6 samples (individual vials), a minimum of 420 μl of 20 μM 59Fe is needed since each sample (vial) needs 70 μl of 20 μM 59Fe. It is recommended that the amount of 59Fe solution to be used is calculated assuming an extra 1.5 samples. For this example, prepare 7.5 x 70 μl, or 525 μl of 59Fe stock. This precaution ensures that, even with pipetting errors, there will be enough to make standards.
Thus, for 525 μl of 20 μM 59Fe = 3.03 μl of 3.47 mM 59Fe stock (see Note below) plus 521.97 μl ddH2O. In each vial, therefore, 70 μl of 20 μM 59Fe stock in 7 ml uptake buffer = 0.2 μM 59Fe.
Make serial dilutions of the 20 μM 59Fe stock to make the 2 and 0.2 μM stocks used for the standards.
For example, 100 μl 20 μM 59Fe + 900 μl ddH2O = 2 μM 59Fe.Notes:

Do not dilute the entire stock vial of radionuclide received from the vendor! Make up stock working solution only as needed.

Calculate the [59Fe] in the radionuclide received from the vendor. An example is given for product number NEZ037001MC from Perkin Elmer Life Sciences.

Divide the concentration by the specific activity. This gives the concentration in mg/ml [(mCi/ml)/(mCi/mg) = mg/ml].
For example, If Perkin Elmer ships 0.5 mCi 59Fe having a specific activity of 72.87 mCi/mg with a concentration of 49.36 mCi/ml, the [59Fe] is determined in the following way:
(49.36 mCi/ml)/(72.87 mCi/mg) = 0.67737 mg/ml
(0.67737 mg/ml)/(59 g) = 0.01148 mmoles of Fe/ml
(0.01148 mmol/ml) (1,000 ml/L) = 11.48 mM = [59Fe]
This concentration is as of the stock date (the date the batch of 59Fe was made). In order to determine the concentration as of the date of your experiment, count the number of days between the stock date and your experiment date. Using an 59Fe decay table, find the corresponding decay factor. Multiply the decay factor by the stock date calibration. This gives you the [59Fe] in the stock vial as of the date of your experiment.
To determine the volume of liquid in the 59Fe stock vial, divide the amount of iron you ordered (i.e., 500 μCi) by the concentration (mCi/ml) on the date one week after the iron was shipped.

Acid-wash glassware treatment
Soak glassware in HNO3 (3-6 h)Note: This process could be performed overnight.
Rinse 3 times in deionized water
Fill 1/3 of the glassware with water and autoclave (cap with aluminum foil)
Pour out water and wash more 3 times with water
Refill glassware with water (1/3 full) and autoclave again (cap with aluminum foil)
Pour water out just prior to use

Acknowledgments

The authors would like to thank to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Comissāo de Avaliação de Pessoal de Nível Superior (CAPES) and Fundação de Amparo a Pesquisa do Estado de Goiás (FAPEG). Development of this protocol was supported by a grant from the National Institutes of Health (US) DK053820 to DJK.

Please login to post your questions/comments. Your questions will be directed to the authors of the protocol. The authors will be requested to answer your questions at their earliest convenience. Once your questions are answered, you will be informed using the email address that you register with bio-protocol.
You are highly recommended to post your data including images for the troubleshooting.

You are highly recommended to post your data (images or even videos) for the troubleshooting. For uploading videos, you may need a Google account because Bio-protocol uses YouTube to host videos.